Process for the production of an aluminum hydroxide Al(OH)3 having rounded grain surfaces

ABSTRACT

A process for the production of an aluminum hydroxide with rounded grain surfaces. An alkaline solution of a specific composition derived from the Bayer process is inoculated and adsorptively precipitated with an aluminum hydroxide with a specific grain distribution. The aluminum hydroxide obtained according to the new process is especially suitable for the flameproof finishing of polyesters.

This is a continuation application of Ser. No. 08/370,813, filed on Jan.10, 1995, which is a continuation application of Ser. No. 08/120,785,filed on Sep. 15, 1993 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a new process for the production of an aluminumhydroxide [Al(OH)₃ ] having rounded grain surfaces as well as flameproofplastic materials containing Al(OH)₃ as filler.

2. Background Art

Aluminum hydroxide is a filler long known for flameproofing polymericmaterials. The Al(OH)₃ obtained from the Bayer process is mainly used asa filler either directly or optionally after a grinding process.However, the grain surfaces of these hydroxides are very irregular,sharp-edged and fissured and, as a result, produce a rough surface inthe correspondingly filled plastic molded articles,

In the incorporation of ground Bayer Al(OH)₃ into plastic, unforeseeableviscosity variations were also observed that raise substantial problemsin the manufacturing process. To counteract these difficulties, effortshave been made to coat the surface of the Al(OH)₃ particles with silanes(German Patent No. 2,743,682) . Besides the additional effort and theadditional costs that result from the separate coatings, the processingproperties of the so-treated Al(OH)₃ are still unsatisfactory.

According to European Published Patent Application No. 011,667, it wasthen attempted to convert the Al(OH)₃ resulting from the Bayer processwhich is present as agglomerates of individual crystals into a heatedaluminum subsaturated Bayer alkaline solution and to stir it for 1 to 25hours, whereupon the solid is separated. The original agglomerates areseparated by this treatment at their grain boundaries. Thus, theindividual primary crystals undergo a rounding; moreover, the resultingrounded Al(OH)₃ particles have practically no fine-grain portion. ThisAl(OH)₃ shows a very good viscosity behavior in plastic, and moreoverthe resulting surfaces the filled plastic molded articles are smooth.However, a substantial drawback in this process is that contaminantsenclosed in the original Bayer Al(OH)₃ are precipitated onto theresulting Al(OH)₃ which leads to discoloration when using unsaturatedpolyester resins (UP resins).

It was proposed according to European Published Patent Application No.407,595 that Al(OH)₃ aggregates derived from the Bayer process bephysically deagglomerated, e.g., by centrifugation or by grinding in aball mill. But damages to the grain surfaces occur through the physicaltreatment which has a detrimental effect on the processing properties inthe plastic. Moreover, such a treatment involves considerable expense.

BROAD DESCRIPTION OF THE INVENTION

The main objective of the invention is to provide a process thateliminates the above-mentioned drawbacks of the known processes, andthat produces Al(OH)₃ which has rounded grain surfaces and that hasexcellent processing properties in plastics. Other objectives andadvantages of the invention are set out herein or are obvious herefromto one skilled in the art.

The objectives and advantages of the invention are achieved by theprocesses, materials and compositions of the invention.

The invention involves a process for the production of aluminumhydroxide Al(OH)₃ having rounded gain surfaces. In the process, analkaline solution derived from the Bayer process, having a molar ratioof Na₂ O to Al₂ O₃ of 2.0 to 2.3, is inoculated with an aluminumhydroxide, having a gain diameter in the 50 percent range d₅₀ of 5 to 25μm, in the 90 percent range d₉₀ of 10 to 50 μm, in the 10 percent ranged₁₀ of 1.0 to 4.5 μm, and then is precipitated with the resulting solidsubsequently being filtered off.

Preferably the aluminum hydroxide used for the seeding is obtained bygrinding and then grading an aluminum hydroxide derived from the Bayerprocess, having a grain diameter in the 50 percent range d₅₀ of 30 to100 μm, and consisting of primary crystals having a grain diameter inthe 50 percent range d₅₀ of 5 to 25 μm. Preferably the seed material isadded to the alkaline solution at a temperature between 60° and 75° C.Preferably the seed material is added to the alkaline solution in anamount of 25 to 500 g/l. Preferably, after seeding, the

precipitation takes place until a molar ratio of Na₂ O to Al₂ O₃ of 2.6to 3.1 has been reached.

The invention also involves aluminum hydroxide having rounded grainsurface with a grain diameter in the 50 percent range d₅₀ of 5 to 25 μm,in the 90 percent range d₉₀ of 10 to 50 μm, and in the 10 percent ranged₁₀ of 1.0 to 4.5 μm, a BET surface of 0.3 to 1.3 m² /g and a surfaceroughness coefficient of 1.1 to 1.5, produced according to the inventionprocess.

The invention involves plastic materials, molding compounds or moldedparts composed of polymeric materials which contain the aluminumhydroxide, having rounded grain surfaces, of the invention.

The invention further includes compositions of polymers containing theinvention aluminum hydroxide as a flame-retarding filler and a processof preparing such compositions.

DETAILED DESCRIPTION OF THE INVENTION

To obtain the Al(OH)₃ used in the process according to the invention, anAl(OH)₃ derived from the Bayer process is ground in a way which isconventional to one skilled in the art, e.g., in a ball mill, and gradedwith a suitable screening device.

This Bayer Al(OH)₃ is originally available as an agglomerate with aparticle size in the 50 percent range d₅₀ of 30 to 100 μm, usually of 50to 70 μm. Correspondingly the primary crystals have a particle size inthe 50 percent range d₅₀ of 2 to 25 μm, suitably of 5 to 25 μm. Thespecific surface according to BET is suitably between 0.1 and 0.5 m² /g,which results in a surface roughness of 2 to 6 (expressed as thequotient of the measured specific surface according to BET and thecalculated surface based upon the assumption of the ideal ball shape ofthe particles).

The grinding and grading of the Bayer Al(OH)₃ is carried out so thatAl(OH)₃ particles result which have the following specifications:

grain diameter in the 50 percent range d₅₀ of 5 to 25 μm,

grain diameter in the 10 percent range d₁₀ of 1.0 to 4.5 μm,

gain diameter in the 90 percent range d₉₀ of 10 to 50 μm,

specific surface, according to BET, of 1 to 3 m² /g,

surface roughness of 2 to 6.

With this ground and graded Al(OH)₃, an alkaline solution which isobtained according to the invention from the Bayer process and which hasa molar ratio of Na₂ O to Al₂ O₃ of 2.0 to 2.3, is inoculated. Thealkaline solution having a suitable composition can be obtained, forexample, by mixing in equal parts a clear alkaline solution (K-solution)having a temperature of about 95° C., which is derived from the Bayerprocess and which has a Na₂ O content of about 140 g/l and an Al₂ O₃content of 150 g/l, with a so-called P alkaline solution having atemperature of, e.g., 45° C., which was obtained after thecrystallization of the Al(OH)₃, and which has a Na₂ O content of about150 g/l and an Al₂ O₃ content of about 85 g/l. Depending on therespective mixture ratio, the obtained alkaline solution has atemperature of 45° to 95° C.

Preferably the seed material is added when the alkaline solution has ata temperature of 60° to 75° C.

The amount of seed material depends on the desired grain distribution inthe desired product. Suitably the seed material is used in an amount of25 to 500 g/l, preferably in an amount of 50 to 150 g/l.

After addition of the seed material the suspension is

precipitated, which can take place at constant temperature, by normalcooling or by cooling according to a specific temperature profile.Suitably the precipitation takes place up until a molar ratio of Na₂ Oto Al₂ O₃ of 2.6 to 3.1 has been reached. As a rule this process takes24 to 60 hours. The suspension obtained after the

precipitation is filtered in ways which are conventional to one skilledin the art.

The resultant Al₂ (OH)₃ is distinguished by the following properties:

grain diameter in the 50 percent range d₅₀ of 5 to 25 μm,

grain diameter in the 10 percent range d₁₀ of 1.0 to 4.5 μm,

grain diameter in the 90 percent range d₉₀ of 10 to 50 μm,

specific surface, according to BET, of 0.3 to 1.3 m² /g,

surface roughness of 1.1 to 1.5.

This Al₂ (OH)₃ produced according to the process of the invention,because of its excellent viscosity behavior, can be worked problem-freeinto plastics, preferably into thermosetting plastic, such as,unsaturated polyester resins (UP resins). Thus, extraordinarily highfilling ratios of filled plastics with very good property profiles canbe achieved. The surfaces of suitably produced molded articles aresmooth. The following examples illustrate the invention.

EXAMPLE 1

In an agitation decomposer (8 m³ capacity) a P-alkaline solution at 45°C. with a Na₂ O content of 145 g/l and an Al₂ O₃ content of 88 g/l wasmixed with a K-alkaline solution at 75° C. with a Na₂ O content of 139g/l and an Al₂ O₃ content of 155 g/l. 5 m³ of a mixed alkaline solutionwith a molar ratio of Na₂ O to Al₂ O₃ of 2.01 resulted. This mixedalkaline solution was heated to approximately 70° C. and mixed with 50kg/m³ of a ground Bayer Al(OH)₃ with d₅₀ of 9 to 13 μm, die of 1.5 μm,d₉₀ of 19 μm and a specific surface according to BET of 2.21 m² /g asthe seed material. The resulting temperature was 67° C. The

precipitation process was now run according to the following temperatureprofile: cooled from 67° to 60° C. in 12 hours and then stirred for 36hours at this temperature. After this precipitation time, a molar ratioof Na₂ O to Al₂ O₃ of 2.96, which corresponded to a Al₂ O₃ yield of 37kg/m³, was reached. The total product yield (including the amount ofseed material) of this absorptive precipitation process was 540 kg ofaluminum hydroxide. The precipitated suspension was filtered on a bandfilter (approximately 15 m² filter surface) and the moist aluminumhydroxide was dried on a contact drier. The resultant Al(OH)₃ had agrain diameter in the 50 percent range of 8.7 μm, in the 10 percentrange of 1.6 μm and in the 90 percent range of 19 μm, a specific surfaceaccording to BET of 1.18 m² /g and a surface roughness of 1.31. Theviscosity of each of (a) the initial product, (b) the Al(OH)₃ accordingto the invention which was obtained according to Example 1 and (c) theproduct which was obtained according to the process of EuropeanPublished Patent Application No. 011,667 (Apyral 4 of VereinigteAluminum Werke) for comparison, was tested in a acrylic resin (Modar 826HT, ICI Acrylix).

Measurement conditions: 170 parts of Al(OH)₃ per 100 parts of resin,Brookfield HBT viscosimeter, Spindel 2, 50 rpm, 20° C.

Results: (a) 4000 mPaS (b) 2400 mPaS (invention) (c) 2600 mPaS(comparison).

The comparison of the degree of whiteness between product (b) andproduct (c) , measured according to DIN 53163 (Erepho, 475 nm, StandardBaSO4), showed for: (b) 91.0 percent (invention) and (c) 86.2 percent(comparison).

EXAMPLE 2

In an agitation decomposer (8 m³ capacity) a P-alkaline solution at 48°C. with a Na₂ O content of 152 g/l and an Al₂ O₃ content of 78 g/l wasmixed with a K-alkaline solution at 82° C. with a Na₂ O content of 144g/l and an Al₂ O₃ content of 164 g/l 5 m³ of a mixed alkaline solutionwith a molar ratio of Na₂ O to Al₂ O₃ of 2.02 resulted. This mixedalkaline solution was heated to approximately 63° C. and mixed with 150kg/m³ of a ground Bayer Al(OH)₃ with d₅₀ of 9 to 13 μm, d₁₀ of 15 μm andd₉₀ of 19 μm and a specific surface according to BET of 2.21 m² /g asthe seed material. The resulting temperature was 60° C. The

precipitation process was run at a steady temperature of 60° C. for 48hours. After this precipitation time, a molar ratio of Na₂ O to Al₂ O₃of 3.1 was reacted corresponding to a Al₂ O₃ yield of 37 kg/m³. Thetotal product yield of this precipitation process (including the seedmaterial amount) was 1063 kg of aluminum hydroxide. The precipitatedsuspension was filtered on a band filter (approximately 15 m² filtersurface) and the moist aluminum hydroxide was dried on a contact drier.The resultant Al(OH)₃ had a grain diameter in the 50 percent range of6.8 μm, in the 10 percent range of 1.5 μm, and in the 90 percent rangeof 15 μm, a specific surface according to BET of 1.21 m² /g and asurface roughness of 1.2. The viscosity was measured according toExample 1 and the results were:

Results: (a) 4000 mPaS (b) 1600 mPaS (invention) (c) 2600 mPaS(comparison)

EXAMPLE 3

In a laboratory agitation decomposer (10 1 capacity) a P-alkalinesolution at 42° C. with a Na₂ O content of 148 g/l and an Al₂ O₃ contentof 101 g/l was mixed with a K-alkaline solution at 79° C. with a Na₂ Ocontent of 138 g/l and an Al₂ O₃ content of 154 g/l . 8 1 of a mixedalkaline solution with a molar ratio of Na₂ O to Al₂ O₃ of 2.04resulted. This mixed alkaline solution was heated to about 70° C. andmixed with 100 g/l of a ground Bayer Al(OH)₃ with d₅₀ of 8.26 μm, die of1.41 μm and d₉₀ of 19.8 μm, and a specific surface according to BET of2.03 m² /g and a surface roughness of 2.44 as the seed material. Theresulting temperature was 67° C. The precipitation process was then runaccording to the following temperature profile: cooled to 60° C. in 24hours and then stirred for 26 hours at this temperature. After thisprecipitation time, a molar ratio of Na₂ O to Al₂ O₃ of 2.78 was reachedcorresponding to a Al₂ O₃ yield of 33.5 kg/m³. The total product yield(including the seed material amount) was 1.21 kg of aluminum hydroxide.The precipitated suspension was filtered on a laboratory nutsch filter(about 500 m²) and dried in a drying oven at 105° C. The resultantAl(OH)₃ had a grain diameter in the 50 percent range of 8.14 μm, in the10 percent range of 1.6 μm and in the 90 percent range of 19.6 μm, aspecific surface according to BET of 0.89 m² /g and a surface roughnessof 1.21. The viscosity of each of initial product (a) and of product (b)obtained according to Example 3 were tested in an unsaturated polyesterresin (Synolite W20, DSM) .

Measurement conditions: 175 parts of Al(OH)₃ per 100 parts of resin,Brookfield HBT viscosimeter, Spindel 3, 5 rpm, 23° C.

Results: (a) 141.6 PaS (b) 57.3 PaS

What is claimed is:
 1. A flame-retarded polymeric composition comprisinga polymeric material which contains aluminum hydroxide particles, thealuminum hydroxide particles having rounded grain surfaces, theparticles of the aluminum hydroxide having a grain diameter in the 50percent range d₅₀ of 5 to 25 μm, in the 90 percent range d₉₀ of 10 to 50μm, and in the 10 percent range d₁₀ of 1.0 to 4.5 μm, a BET surface of0.3 to 1.3 m² /g, and a surface roughness coefficient of 1.1 to 1.5, thealuminum hydroxide particles being produced by the process comprisinginoculating an alkaline solution which is derived from the Bayer processand which has a molar ratio of Na₂ O to Al₂ O₃ of 2.0 to 2.3, with 25 to500 g/l of an aluminum hydroxide, the particles of which have a graindiameter in the 50 percent range d₅₀ of 5 to 25 μm, in the 90 percentrange d₉₀ of 10 to 50 μm and in the 10 percent range d₁₀ of 1.0 to 4.5μm, there being sufficient agitation of the alkaline solution to keepthe inoculated particles in suspension, and the inoculated alkalinesolution being maintained a sufficient amount of time to achievesufficient particle growth to provide solid aluminum hydroxide withrounded grain surfaces, the particles of which have a grain diameter inthe 50 percent range d₅₀ of 5 to 25 μm, in the 90 percent range d₉₀ of10 to 50 μm and in the 10 percent range d₁₀ of 1.0 to 4.5μ, and then isprecipitated at constant temperature or by normal cooling or by coolingat a specific temperature profile of said alkaline solution until amolar ratio of Na₂ O to Al₂ O₃ of 2.6 to 3.1 has been reached in saidalkaline solution, and then said solid aluminum hydroxide with roundedgrain surfaces is filtered off.
 2. A process of preparing aflame-retarded polymeric composition comprising incorporating aluminumhydroxide particles as a flame-retarding filler into a polymericmaterial, the aluminum hydroxide particles having rounded grainsurfaces, the particles of the aluminum hydroxide having a graindiameter in the 50 percent range d₅₀ of 5 to 25 μm, in the 90 percentrange d₉₀ of 10 to 50 μm, and in the 10 percent range d₁₀ of 1.0 to 4.5μm, a BET surface of 0.3 to 1.3 m² /g, and a surface roughnesscoefficient of 1.1 to 1.5, the aluminum hydroxide particles beingproduced by the process comprising inoculating an alkaline solutionwhich is derived from the Bayer process and which has a molar ratio ofNa₂ O to Al₂ O₃ of 2.0 to 2.3, with 25 to 500 g/l of an aluminumhydroxide, the particles of which have a grain diameter in the 50percent range d₅₀ of 5 to 25 μm, in the 90 percent range d₉₀ of 10 to 50μm and in the 10 percent range d₁₀ of 1.0 to 4.5 μm, there beingsufficient agitation of the alkaline solution to keep the inoculatedparticles in suspension, and the inoculated alkaline solution beingmaintained a sufficient amount of time to achieve sufficient particlegrowth to provide solid aluminum hydroxide with rounded grain surfaces,the particles of which have a grain diameter in the 50 percent range d₅₀of 5 to 25 μm, in the 90 percent range d₉₀ of 10 to 50 μm and in the 10percent range d₁₀ of 1.0 to 4.5μ, and then is precipitated at constanttemperature or by normal cooling or by cooling at a specific temperatureprofile of said alkaline solution until a molar ratio of Na₂ O to Al₂ O₃of 2.6 to 3.1 has been reached in said alkaline solution, and then saidsolid aluminum hydroxide with rounded grain surfaces is filtered off.